The present invention relates a microwave testing dummy load method and apparatus.
A conventional dummy load of this type having a large capacity of 1 kW or more requires a large resistive element. For this reason, an oil- or water-cooled dummy load is used where a metal film is deposited on the surface of a porcelain member, and the member is dipped in an insulating oil or water. However, this dummy load is expensive and hence is not practical.
A dry dummy load, which can be manufactured at a low cost and demands no maintenance, has a structure in which microwave power is input from an input external conductor 5 to a resistive element 14e on a dielectric substrate 11, as shown in FIGS. 4A and 4B. The resistive element 14e is formed by depositing a metal film such as a carbonyl iron powder or graphite film on the surface of an insulator with a low thermal resistance, e.g., beryllia or alumina. Reference numeral 13 denotes a ground conductor.
A dummy load apparatus is disclosed in Japanese Patent Laid-Open No. 61-147601 (JP '601) as an apparatus in which input microwave power is distributed to a plurality of termination resistors to reduce the load on each termination resistor.
In the dummy load apparatus disclosed in (JP '601), hybrid circuits 15, 16, and 17 including phase adjusting resistors 15R, 16R and 17R respectively, are formed on a dielectric substrate 11, and the hybrid circuits 15 and 16 and the hybrid circuits 15 and 17 are connected to each other through resistive center conductors 12a and 12b, respectively, as shown in FIGS. 5A and 5B. Output center conductors 12e and 12f of the hybrid circuit 16 are connected to termination resistors 14c and 14d. Output center conductors 12c and 12d of the hybrid circuit 17 are connected to termination resistors 14a and 14b. The termination resistors 14a to 14d are connected to a common ground conductor 13.
For example, a dummy load of 3W can therefore be formed by setting the rated power of each of the termination loads 14a to 14d to EQU 3W.times.1/4=750 mW
In addition, if the termination resistors are flat resistors, the allowable power per unit area is a maximum of 30 mW/mm. The area of one termination resistor is therefore given by EQU 750.div.30=25 mm.sup.2
In the former dry dummy load apparatus described above, a beryllia porcelain member having a very low thermal resistance is mainly used as a porcelain member on which a resistive film is formed. However, a beryllia porcelain member is expensive, requires a resistive film having a large area in proportion to the allowable power, and suffers a deterioration in impedance characteristics because an increase in area leads to an increase in stray capacitance. Owing to these problems, as a dummy load apparatus of this type, an apparatus having a power capacity of about 500 W at most can be commercially available at present in consideration of limitations associated with manufacturing techniques, price, temperature rise, and the like. Even if a heat radiation plate is directly mounted on this porcelain member , only a dummy load apparatus having a power capacity of about 1 kW at most can be used in practice.
In the latter dummy load apparatus designed to distribute power to a plurality of termination resistors, the center conductors 12 and 12a to 12f are formed as thin films on the dielectric substrate 11, as shown in FIG. 5B. Since this structure is formed without any consideration of a temperature rise, only a power capacity of several watts can be obtained.